The present invention relates generally to masthead units for base stations used in cellular communication networks. More particularly, a self-contained modular masthead unit and methods for changing the cell size by swapping masthead units are described.
The popularity of cellular telephones has been increasing dramatically in recent years. With the increased demand for cellular services, there has been an explosive growth in the infrastructure that is required to support these cellular services. Most cellular networks are organized substantially as illustrating in FIG. 1. As seen therein, a mobile switching center (MSC) 10 communicates with a plurality of base station controllers 12 which in turn each communicate with one or more base stations 14. The base stations 14 are directly coupled to one or more antennas (transceivers) 16 and are responsible for transmitting or receiving the radio signals that are used to support cellular communications. The mobile switching center 10 operates as the nerve center for the entire network and communicates with the base station controllers (BSC) 12 by an established protocol. There are a variety of different protocols that have been defined for cellular services, as for example, the GSM (Global Systems for Mobile Communications) protocol, the CDMA (Code Division Multiple Access) and the TDMA (Time Division Multiple Access) protocols. These various protocols dictate the nature of the communications between the MSC, the BSCs, and the BTSs and are well known to those skilled in the art.
Conventional base station controllers are primarily responsible for dictating the size of the associated cell. That is, the area that is covered by a particular base station. There are no fixed specifications as to the size of the cells, but in current usage, it is common to refer to macro cells, mini cells, and micro cells. The range of the various cells tends to vary with their size and by way of example in current usage, macro cells typically have antennas that output on the order of 20-50 watts of energy and tend to have ranges on the order of 5-40 kilometers. Mini cells typically have power outputs on the order of 10 watts and corresponding ranges in the vicinity of 2-5 kilometers. Micro cells typically have power consumption on the order of 2-8 watts with ranges of less than a kilometer or so. Of course as signal processing capabilities in antenna designs improve, the distinctions between the various sizes blurs but in concept, the cell size may always be varied.
One problem frequently encountered by systems having relatively larger cell sizes is that the antenna must be placed on a tower in order to obtain the desired range. Referring next to FIG. 2, one conventional tower arrangement will be briefly described. In the illustrated arrangement, the antenna 16 is mounted on a tower structure 20 while the associated base station 14 is located in the building structure at the base of the tower 20. If the tower 20 is relatively tall, a relatively long feed cable 22 must be provided between the base station 14 and the antenna 16. Generally the feed cable 22 includes a pair of coax cables with one of the coax cables (a transmit line) being arranged to carry the transmit signal and one of the coax cables (a receive line) being arranged to carry the receive signal.
A relatively long feed cable 22 presents a number of difficulties in terms of both signal intensity losses and the introduction of noise to the received signal. One approach to reducing the problems encountered by long feed lines is to provide a masthead unit 24 that is mounted on the tower 20 at a position relatively closer to the antenna 16. The masthead unit typically includes a duplexer that is arranged to apply the transmitted signals to the antenna and to split the received signals onto a receive line. The split received signals are then passed through a low noise amplifier which amplifies the received signals making them substantially less susceptible to degradation by noise picked up by the feed cable 22.
When the feed cable is particularly long there may be relatively significant power losses in the transmit signal carried on the transmit line as well. Accordingly, some designs have incorporated a linear power amplifier into the masthead design as well. Such power amplifiers are fed by a power supply located in the base station.
As cellular technology improves and its popularity increases there are increased needs to alter the size and/or shape of particular cells. In some cases it is desirable to increase the size of a cell to provide improved range. At the same time, in other situations it is desirable to reduce the size of a cell to permit the introduction of additional cells in order to handle higher usage. Although the present designs work well, they are not particularly modular in that if it is desirable to change the size of a cell for any reason, it is necessary to replace the entire base station 14. Since base stations are relatively expensive units it would be desirable to provide a base station architecture which permits the base station hardware to be reused even if the cell size or cell geometry needs to change.